Mpox: How deadly is the virus and what treatments are available?

Mpox, formerly known as monkeypox, is driving an ongoing outbreak in the Democratic Republic of the Congo (DRC) and neighbouring countries. Surging cases are partly caused by a new variant that is thought to be more lethal than the version of the virus behind the global mpox outbreak in 2022. But there are treatments that could help.

How deadly is mpox?

Studies show that the fatality rate among people who catch the clade I variant of mpox, which is driving the ongoing outbreak, ranges from around 1 to 11 per cent. The variation in reported fatality rates is probably due to differences in the populations that have fallen ill, and to problems with disease surveillance, says Lilith Whittles at Imperial College London.

For example, infants and children, who have less developed immune systems, may be more likely to develop severe – and potentially deadly – infections compared with adults. And people with suppressed immune systems, such as those with HIV, are also more vulnerable, she says.

What’s more, people in some regions have little access to healthcare, and thus limited mpox surveillance. As a result, only the most severe cases end up being detected in healthcare clinics, while milder cases are missed, making fatality rates seem higher than they really are. More frequent misdiagnoses of mpox symptoms as another disease, such as measles or chickenpox, will also leave more cases undetected, says Whittles.

When people do die of mpox, it is due to complications such as sepsis, where the infection ends up in the bloodstream and causes organ failure, or lung damage due to inflammation caused by the mpox virus, says Piero Olliaro at the University of Oxford.

What mpox treatments do we have?

In the DRC and nearby countries at the centre of the ongoing outbreak, treatments specifically for mpox are largely unavailable. Instead, doctors focus on treating symptoms, which typically last for two to four weeks. That includes easing fevers and headaches with paracetamol (acetaminophen), or cleaning skin lesions to prevent bacterial infections, says Jean Claude Udahemuka at the University of Rwanda.

Elsewhere, in the UK and US, doctors can use the antiviral drug tecovirimat to treat people with severe forms of mpox. Originally developed to treat smallpox, its use against mpox is based on animal studies in which it improved survival rates compared with a placebo. Tecovirimat works by binding to a protein, found on the surface of both mpox and smallpox, which the viruses use to release themselves from an infected cell and spread to other cells.

Doctors in the US and UK can also treat mpox with other antivirals, including brincidofovir and cidofovir, which have been shown to protect mice from lethal doses of the mpox virus. Brincidofivir and cidofovir both interfere with an enzyme that the virus uses to replicate its genome.

And another treatment, known as VIGIV, involves injecting antibodies against smallpox – collected from people who have received smallpox vaccines – into those infected with mpox. This boosts the immune response against viruses.

How effective are mpox treatments in people?

While animal studies suggest these treatments work against mpox, their effectiveness in people is unknown. Initial results from a recent randomised controlled trial – the best kind of medical evidence – in the DRC suggest that tecovirimat doesn’t speed up healing of painful lesions in children and adults infected with the clade I variant of mpox, which is driving the ongoing outbreak.

Despite this, researchers found that the mpox mortality rate of participants who received the antiviral was 1.7 per cent, an improvement on the 3.6 per cent mortality rate typically seen in the DRC. However, this could be partially explained by the fact that participants enrolled in the trial were being closely cared for in hospital, says Olliaro.

Ultimately, better treatments and a better grasp of how deadly mpox can be will be essential to protect people, especially those in the DRC, against the ongoing outbreak, says Lucille Blumberg at the University of Pretoria in South Africa. “There’s lots of work to do,” she says.